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1 <html>
2 <head>
3 <title>pcrepattern specification</title>
4 </head>
5 <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6 <h1>pcrepattern man page</h1>
7 <p>
8 Return to the <a href="index.html">PCRE index page</a>.
9 </p>
10 <p>
11 This page is part of the PCRE HTML documentation. It was generated automatically
12 from the original man page. If there is any nonsense in it, please consult the
13 man page, in case the conversion went wrong.
14 <br>
15 <ul>
16 <li><a name="TOC1" href="#SEC1">PCRE REGULAR EXPRESSION DETAILS</a>
17 <li><a name="TOC2" href="#SEC2">NEWLINE CONVENTIONS</a>
18 <li><a name="TOC3" href="#SEC3">CHARACTERS AND METACHARACTERS</a>
19 <li><a name="TOC4" href="#SEC4">BACKSLASH</a>
20 <li><a name="TOC5" href="#SEC5">CIRCUMFLEX AND DOLLAR</a>
21 <li><a name="TOC6" href="#SEC6">FULL STOP (PERIOD, DOT)</a>
22 <li><a name="TOC7" href="#SEC7">MATCHING A SINGLE BYTE</a>
23 <li><a name="TOC8" href="#SEC8">SQUARE BRACKETS AND CHARACTER CLASSES</a>
24 <li><a name="TOC9" href="#SEC9">POSIX CHARACTER CLASSES</a>
25 <li><a name="TOC10" href="#SEC10">VERTICAL BAR</a>
26 <li><a name="TOC11" href="#SEC11">INTERNAL OPTION SETTING</a>
27 <li><a name="TOC12" href="#SEC12">SUBPATTERNS</a>
28 <li><a name="TOC13" href="#SEC13">DUPLICATE SUBPATTERN NUMBERS</a>
29 <li><a name="TOC14" href="#SEC14">NAMED SUBPATTERNS</a>
30 <li><a name="TOC15" href="#SEC15">REPETITION</a>
31 <li><a name="TOC16" href="#SEC16">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a>
32 <li><a name="TOC17" href="#SEC17">BACK REFERENCES</a>
33 <li><a name="TOC18" href="#SEC18">ASSERTIONS</a>
34 <li><a name="TOC19" href="#SEC19">CONDITIONAL SUBPATTERNS</a>
35 <li><a name="TOC20" href="#SEC20">COMMENTS</a>
36 <li><a name="TOC21" href="#SEC21">RECURSIVE PATTERNS</a>
37 <li><a name="TOC22" href="#SEC22">SUBPATTERNS AS SUBROUTINES</a>
38 <li><a name="TOC23" href="#SEC23">ONIGURUMA SUBROUTINE SYNTAX</a>
39 <li><a name="TOC24" href="#SEC24">CALLOUTS</a>
40 <li><a name="TOC25" href="#SEC25">BACKTRACKING CONTROL</a>
41 <li><a name="TOC26" href="#SEC26">SEE ALSO</a>
42 <li><a name="TOC27" href="#SEC27">AUTHOR</a>
43 <li><a name="TOC28" href="#SEC28">REVISION</a>
44 </ul>
45 <br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br>
46 <P>
47 The syntax and semantics of the regular expressions that are supported by PCRE
48 are described in detail below. There is a quick-reference syntax summary in the
49 <a href="pcresyntax.html"><b>pcresyntax</b></a>
50 page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
51 also supports some alternative regular expression syntax (which does not
52 conflict with the Perl syntax) in order to provide some compatibility with
53 regular expressions in Python, .NET, and Oniguruma.
54 </P>
55 <P>
56 Perl's regular expressions are described in its own documentation, and
57 regular expressions in general are covered in a number of books, some of which
58 have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
59 published by O'Reilly, covers regular expressions in great detail. This
60 description of PCRE's regular expressions is intended as reference material.
61 </P>
62 <P>
63 The original operation of PCRE was on strings of one-byte characters. However,
64 there is now also support for UTF-8 character strings. To use this, you must
65 build PCRE to include UTF-8 support, and then call <b>pcre_compile()</b> with
66 the PCRE_UTF8 option. How this affects pattern matching is mentioned in several
67 places below. There is also a summary of UTF-8 features in the
68 <a href="pcre.html#utf8support">section on UTF-8 support</a>
69 in the main
70 <a href="pcre.html"><b>pcre</b></a>
71 page.
72 </P>
73 <P>
74 The remainder of this document discusses the patterns that are supported by
75 PCRE when its main matching function, <b>pcre_exec()</b>, is used.
76 From release 6.0, PCRE offers a second matching function,
77 <b>pcre_dfa_exec()</b>, which matches using a different algorithm that is not
78 Perl-compatible. Some of the features discussed below are not available when
79 <b>pcre_dfa_exec()</b> is used. The advantages and disadvantages of the
80 alternative function, and how it differs from the normal function, are
81 discussed in the
82 <a href="pcrematching.html"><b>pcrematching</b></a>
83 page.
84 </P>
85 <br><a name="SEC2" href="#TOC1">NEWLINE CONVENTIONS</a><br>
86 <P>
87 PCRE supports five different conventions for indicating line breaks in
88 strings: a single CR (carriage return) character, a single LF (linefeed)
89 character, the two-character sequence CRLF, any of the three preceding, or any
90 Unicode newline sequence. The
91 <a href="pcreapi.html"><b>pcreapi</b></a>
92 page has
93 <a href="pcreapi.html#newlines">further discussion</a>
94 about newlines, and shows how to set the newline convention in the
95 <i>options</i> arguments for the compiling and matching functions.
96 </P>
97 <P>
98 It is also possible to specify a newline convention by starting a pattern
99 string with one of the following five sequences:
100 <pre>
101 (*CR) carriage return
102 (*LF) linefeed
103 (*CRLF) carriage return, followed by linefeed
104 (*ANYCRLF) any of the three above
105 (*ANY) all Unicode newline sequences
106 </pre>
107 These override the default and the options given to <b>pcre_compile()</b>. For
108 example, on a Unix system where LF is the default newline sequence, the pattern
109 <pre>
110 (*CR)a.b
111 </pre>
112 changes the convention to CR. That pattern matches "a\nb" because LF is no
113 longer a newline. Note that these special settings, which are not
114 Perl-compatible, are recognized only at the very start of a pattern, and that
115 they must be in upper case. If more than one of them is present, the last one
116 is used.
117 </P>
118 <P>
119 The newline convention does not affect what the \R escape sequence matches. By
120 default, this is any Unicode newline sequence, for Perl compatibility. However,
121 this can be changed; see the description of \R in the section entitled
122 <a href="#newlineseq">"Newline sequences"</a>
123 below. A change of \R setting can be combined with a change of newline
124 convention.
125 </P>
126 <br><a name="SEC3" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br>
127 <P>
128 A regular expression is a pattern that is matched against a subject string from
129 left to right. Most characters stand for themselves in a pattern, and match the
130 corresponding characters in the subject. As a trivial example, the pattern
131 <pre>
132 The quick brown fox
133 </pre>
134 matches a portion of a subject string that is identical to itself. When
135 caseless matching is specified (the PCRE_CASELESS option), letters are matched
136 independently of case. In UTF-8 mode, PCRE always understands the concept of
137 case for characters whose values are less than 128, so caseless matching is
138 always possible. For characters with higher values, the concept of case is
139 supported if PCRE is compiled with Unicode property support, but not otherwise.
140 If you want to use caseless matching for characters 128 and above, you must
141 ensure that PCRE is compiled with Unicode property support as well as with
142 UTF-8 support.
143 </P>
144 <P>
145 The power of regular expressions comes from the ability to include alternatives
146 and repetitions in the pattern. These are encoded in the pattern by the use of
147 <i>metacharacters</i>, which do not stand for themselves but instead are
148 interpreted in some special way.
149 </P>
150 <P>
151 There are two different sets of metacharacters: those that are recognized
152 anywhere in the pattern except within square brackets, and those that are
153 recognized within square brackets. Outside square brackets, the metacharacters
154 are as follows:
155 <pre>
156 \ general escape character with several uses
157 ^ assert start of string (or line, in multiline mode)
158 $ assert end of string (or line, in multiline mode)
159 . match any character except newline (by default)
160 [ start character class definition
161 | start of alternative branch
162 ( start subpattern
163 ) end subpattern
164 ? extends the meaning of (
165 also 0 or 1 quantifier
166 also quantifier minimizer
167 * 0 or more quantifier
168 + 1 or more quantifier
169 also "possessive quantifier"
170 { start min/max quantifier
171 </pre>
172 Part of a pattern that is in square brackets is called a "character class". In
173 a character class the only metacharacters are:
174 <pre>
175 \ general escape character
176 ^ negate the class, but only if the first character
177 - indicates character range
178 [ POSIX character class (only if followed by POSIX syntax)
179 ] terminates the character class
180 </pre>
181 The following sections describe the use of each of the metacharacters.
182 </P>
183 <br><a name="SEC4" href="#TOC1">BACKSLASH</a><br>
184 <P>
185 The backslash character has several uses. Firstly, if it is followed by a
186 non-alphanumeric character, it takes away any special meaning that character
187 may have. This use of backslash as an escape character applies both inside and
188 outside character classes.
189 </P>
190 <P>
191 For example, if you want to match a * character, you write \* in the pattern.
192 This escaping action applies whether or not the following character would
193 otherwise be interpreted as a metacharacter, so it is always safe to precede a
194 non-alphanumeric with backslash to specify that it stands for itself. In
195 particular, if you want to match a backslash, you write \\.
196 </P>
197 <P>
198 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
199 pattern (other than in a character class) and characters between a # outside
200 a character class and the next newline are ignored. An escaping backslash can
201 be used to include a whitespace or # character as part of the pattern.
202 </P>
203 <P>
204 If you want to remove the special meaning from a sequence of characters, you
205 can do so by putting them between \Q and \E. This is different from Perl in
206 that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in
207 Perl, $ and @ cause variable interpolation. Note the following examples:
208 <pre>
209 Pattern PCRE matches Perl matches
210
211 \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
212 \Qabc\$xyz\E abc\$xyz abc\$xyz
213 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
214 </pre>
215 The \Q...\E sequence is recognized both inside and outside character classes.
216 <a name="digitsafterbackslash"></a></P>
217 <br><b>
218 Non-printing characters
219 </b><br>
220 <P>
221 A second use of backslash provides a way of encoding non-printing characters
222 in patterns in a visible manner. There is no restriction on the appearance of
223 non-printing characters, apart from the binary zero that terminates a pattern,
224 but when a pattern is being prepared by text editing, it is usually easier to
225 use one of the following escape sequences than the binary character it
226 represents:
227 <pre>
228 \a alarm, that is, the BEL character (hex 07)
229 \cx "control-x", where x is any character
230 \e escape (hex 1B)
231 \f formfeed (hex 0C)
232 \n linefeed (hex 0A)
233 \r carriage return (hex 0D)
234 \t tab (hex 09)
235 \ddd character with octal code ddd, or backreference
236 \xhh character with hex code hh
237 \x{hhh..} character with hex code hhh..
238 </pre>
239 The precise effect of \cx is as follows: if x is a lower case letter, it
240 is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
241 Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c; becomes hex
242 7B.
243 </P>
244 <P>
245 After \x, from zero to two hexadecimal digits are read (letters can be in
246 upper or lower case). Any number of hexadecimal digits may appear between \x{
247 and }, but the value of the character code must be less than 256 in non-UTF-8
248 mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
249 hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
250 point, which is 10FFFF.
251 </P>
252 <P>
253 If characters other than hexadecimal digits appear between \x{ and }, or if
254 there is no terminating }, this form of escape is not recognized. Instead, the
255 initial \x will be interpreted as a basic hexadecimal escape, with no
256 following digits, giving a character whose value is zero.
257 </P>
258 <P>
259 Characters whose value is less than 256 can be defined by either of the two
260 syntaxes for \x. There is no difference in the way they are handled. For
261 example, \xdc is exactly the same as \x{dc}.
262 </P>
263 <P>
264 After \0 up to two further octal digits are read. If there are fewer than two
265 digits, just those that are present are used. Thus the sequence \0\x\07
266 specifies two binary zeros followed by a BEL character (code value 7). Make
267 sure you supply two digits after the initial zero if the pattern character that
268 follows is itself an octal digit.
269 </P>
270 <P>
271 The handling of a backslash followed by a digit other than 0 is complicated.
272 Outside a character class, PCRE reads it and any following digits as a decimal
273 number. If the number is less than 10, or if there have been at least that many
274 previous capturing left parentheses in the expression, the entire sequence is
275 taken as a <i>back reference</i>. A description of how this works is given
276 <a href="#backreferences">later,</a>
277 following the discussion of
278 <a href="#subpattern">parenthesized subpatterns.</a>
279 </P>
280 <P>
281 Inside a character class, or if the decimal number is greater than 9 and there
282 have not been that many capturing subpatterns, PCRE re-reads up to three octal
283 digits following the backslash, and uses them to generate a data character. Any
284 subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
285 character specified in octal must be less than \400. In UTF-8 mode, values up
286 to \777 are permitted. For example:
287 <pre>
288 \040 is another way of writing a space
289 \40 is the same, provided there are fewer than 40 previous capturing subpatterns
290 \7 is always a back reference
291 \11 might be a back reference, or another way of writing a tab
292 \011 is always a tab
293 \0113 is a tab followed by the character "3"
294 \113 might be a back reference, otherwise the character with octal code 113
295 \377 might be a back reference, otherwise the byte consisting entirely of 1 bits
296 \81 is either a back reference, or a binary zero followed by the two characters "8" and "1"
297 </pre>
298 Note that octal values of 100 or greater must not be introduced by a leading
299 zero, because no more than three octal digits are ever read.
300 </P>
301 <P>
302 All the sequences that define a single character value can be used both inside
303 and outside character classes. In addition, inside a character class, the
304 sequence \b is interpreted as the backspace character (hex 08), and the
305 sequences \R and \X are interpreted as the characters "R" and "X",
306 respectively. Outside a character class, these sequences have different
307 meanings
308 <a href="#uniextseq">(see below).</a>
309 </P>
310 <br><b>
311 Absolute and relative back references
312 </b><br>
313 <P>
314 The sequence \g followed by an unsigned or a negative number, optionally
315 enclosed in braces, is an absolute or relative back reference. A named back
316 reference can be coded as \g{name}. Back references are discussed
317 <a href="#backreferences">later,</a>
318 following the discussion of
319 <a href="#subpattern">parenthesized subpatterns.</a>
320 </P>
321 <br><b>
322 Absolute and relative subroutine calls
323 </b><br>
324 <P>
325 For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
326 a number enclosed either in angle brackets or single quotes, is an alternative
327 syntax for referencing a subpattern as a "subroutine". Details are discussed
328 <a href="#onigurumasubroutines">later.</a>
329 Note that \g{...} (Perl syntax) and \g&#60;...&#62; (Oniguruma syntax) are <i>not</i>
330 synonymous. The former is a back reference; the latter is a subroutine call.
331 </P>
332 <br><b>
333 Generic character types
334 </b><br>
335 <P>
336 Another use of backslash is for specifying generic character types. The
337 following are always recognized:
338 <pre>
339 \d any decimal digit
340 \D any character that is not a decimal digit
341 \h any horizontal whitespace character
342 \H any character that is not a horizontal whitespace character
343 \s any whitespace character
344 \S any character that is not a whitespace character
345 \v any vertical whitespace character
346 \V any character that is not a vertical whitespace character
347 \w any "word" character
348 \W any "non-word" character
349 </pre>
350 Each pair of escape sequences partitions the complete set of characters into
351 two disjoint sets. Any given character matches one, and only one, of each pair.
352 </P>
353 <P>
354 These character type sequences can appear both inside and outside character
355 classes. They each match one character of the appropriate type. If the current
356 matching point is at the end of the subject string, all of them fail, since
357 there is no character to match.
358 </P>
359 <P>
360 For compatibility with Perl, \s does not match the VT character (code 11).
361 This makes it different from the the POSIX "space" class. The \s characters
362 are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
363 included in a Perl script, \s may match the VT character. In PCRE, it never
364 does.
365 </P>
366 <P>
367 In UTF-8 mode, characters with values greater than 128 never match \d, \s, or
368 \w, and always match \D, \S, and \W. This is true even when Unicode
369 character property support is available. These sequences retain their original
370 meanings from before UTF-8 support was available, mainly for efficiency
371 reasons.
372 </P>
373 <P>
374 The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to the
375 other sequences, these do match certain high-valued codepoints in UTF-8 mode.
376 The horizontal space characters are:
377 <pre>
378 U+0009 Horizontal tab
379 U+0020 Space
380 U+00A0 Non-break space
381 U+1680 Ogham space mark
382 U+180E Mongolian vowel separator
383 U+2000 En quad
384 U+2001 Em quad
385 U+2002 En space
386 U+2003 Em space
387 U+2004 Three-per-em space
388 U+2005 Four-per-em space
389 U+2006 Six-per-em space
390 U+2007 Figure space
391 U+2008 Punctuation space
392 U+2009 Thin space
393 U+200A Hair space
394 U+202F Narrow no-break space
395 U+205F Medium mathematical space
396 U+3000 Ideographic space
397 </pre>
398 The vertical space characters are:
399 <pre>
400 U+000A Linefeed
401 U+000B Vertical tab
402 U+000C Formfeed
403 U+000D Carriage return
404 U+0085 Next line
405 U+2028 Line separator
406 U+2029 Paragraph separator
407 </PRE>
408 </P>
409 <P>
410 A "word" character is an underscore or any character less than 256 that is a
411 letter or digit. The definition of letters and digits is controlled by PCRE's
412 low-valued character tables, and may vary if locale-specific matching is taking
413 place (see
414 <a href="pcreapi.html#localesupport">"Locale support"</a>
415 in the
416 <a href="pcreapi.html"><b>pcreapi</b></a>
417 page). For example, in a French locale such as "fr_FR" in Unix-like systems,
418 or "french" in Windows, some character codes greater than 128 are used for
419 accented letters, and these are matched by \w. The use of locales with Unicode
420 is discouraged.
421 <a name="newlineseq"></a></P>
422 <br><b>
423 Newline sequences
424 </b><br>
425 <P>
426 Outside a character class, by default, the escape sequence \R matches any
427 Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \R is
428 equivalent to the following:
429 <pre>
430 (?&#62;\r\n|\n|\x0b|\f|\r|\x85)
431 </pre>
432 This is an example of an "atomic group", details of which are given
433 <a href="#atomicgroup">below.</a>
434 This particular group matches either the two-character sequence CR followed by
435 LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
436 U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
437 line, U+0085). The two-character sequence is treated as a single unit that
438 cannot be split.
439 </P>
440 <P>
441 In UTF-8 mode, two additional characters whose codepoints are greater than 255
442 are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
443 Unicode character property support is not needed for these characters to be
444 recognized.
445 </P>
446 <P>
447 It is possible to restrict \R to match only CR, LF, or CRLF (instead of the
448 complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
449 either at compile time or when the pattern is matched. (BSR is an abbrevation
450 for "backslash R".) This can be made the default when PCRE is built; if this is
451 the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
452 It is also possible to specify these settings by starting a pattern string with
453 one of the following sequences:
454 <pre>
455 (*BSR_ANYCRLF) CR, LF, or CRLF only
456 (*BSR_UNICODE) any Unicode newline sequence
457 </pre>
458 These override the default and the options given to <b>pcre_compile()</b>, but
459 they can be overridden by options given to <b>pcre_exec()</b>. Note that these
460 special settings, which are not Perl-compatible, are recognized only at the
461 very start of a pattern, and that they must be in upper case. If more than one
462 of them is present, the last one is used. They can be combined with a change of
463 newline convention, for example, a pattern can start with:
464 <pre>
465 (*ANY)(*BSR_ANYCRLF)
466 </pre>
467 Inside a character class, \R matches the letter "R".
468 <a name="uniextseq"></a></P>
469 <br><b>
470 Unicode character properties
471 </b><br>
472 <P>
473 When PCRE is built with Unicode character property support, three additional
474 escape sequences that match characters with specific properties are available.
475 When not in UTF-8 mode, these sequences are of course limited to testing
476 characters whose codepoints are less than 256, but they do work in this mode.
477 The extra escape sequences are:
478 <pre>
479 \p{<i>xx</i>} a character with the <i>xx</i> property
480 \P{<i>xx</i>} a character without the <i>xx</i> property
481 \X an extended Unicode sequence
482 </pre>
483 The property names represented by <i>xx</i> above are limited to the Unicode
484 script names, the general category properties, and "Any", which matches any
485 character (including newline). Other properties such as "InMusicalSymbols" are
486 not currently supported by PCRE. Note that \P{Any} does not match any
487 characters, so always causes a match failure.
488 </P>
489 <P>
490 Sets of Unicode characters are defined as belonging to certain scripts. A
491 character from one of these sets can be matched using a script name. For
492 example:
493 <pre>
494 \p{Greek}
495 \P{Han}
496 </pre>
497 Those that are not part of an identified script are lumped together as
498 "Common". The current list of scripts is:
499 </P>
500 <P>
501 Arabic,
502 Armenian,
503 Balinese,
504 Bengali,
505 Bopomofo,
506 Braille,
507 Buginese,
508 Buhid,
509 Canadian_Aboriginal,
510 Cherokee,
511 Common,
512 Coptic,
513 Cuneiform,
514 Cypriot,
515 Cyrillic,
516 Deseret,
517 Devanagari,
518 Ethiopic,
519 Georgian,
520 Glagolitic,
521 Gothic,
522 Greek,
523 Gujarati,
524 Gurmukhi,
525 Han,
526 Hangul,
527 Hanunoo,
528 Hebrew,
529 Hiragana,
530 Inherited,
531 Kannada,
532 Katakana,
533 Kharoshthi,
534 Khmer,
535 Lao,
536 Latin,
537 Limbu,
538 Linear_B,
539 Malayalam,
540 Mongolian,
541 Myanmar,
542 New_Tai_Lue,
543 Nko,
544 Ogham,
545 Old_Italic,
546 Old_Persian,
547 Oriya,
548 Osmanya,
549 Phags_Pa,
550 Phoenician,
551 Runic,
552 Shavian,
553 Sinhala,
554 Syloti_Nagri,
555 Syriac,
556 Tagalog,
557 Tagbanwa,
558 Tai_Le,
559 Tamil,
560 Telugu,
561 Thaana,
562 Thai,
563 Tibetan,
564 Tifinagh,
565 Ugaritic,
566 Yi.
567 </P>
568 <P>
569 Each character has exactly one general category property, specified by a
570 two-letter abbreviation. For compatibility with Perl, negation can be specified
571 by including a circumflex between the opening brace and the property name. For
572 example, \p{^Lu} is the same as \P{Lu}.
573 </P>
574 <P>
575 If only one letter is specified with \p or \P, it includes all the general
576 category properties that start with that letter. In this case, in the absence
577 of negation, the curly brackets in the escape sequence are optional; these two
578 examples have the same effect:
579 <pre>
580 \p{L}
581 \pL
582 </pre>
583 The following general category property codes are supported:
584 <pre>
585 C Other
586 Cc Control
587 Cf Format
588 Cn Unassigned
589 Co Private use
590 Cs Surrogate
591
592 L Letter
593 Ll Lower case letter
594 Lm Modifier letter
595 Lo Other letter
596 Lt Title case letter
597 Lu Upper case letter
598
599 M Mark
600 Mc Spacing mark
601 Me Enclosing mark
602 Mn Non-spacing mark
603
604 N Number
605 Nd Decimal number
606 Nl Letter number
607 No Other number
608
609 P Punctuation
610 Pc Connector punctuation
611 Pd Dash punctuation
612 Pe Close punctuation
613 Pf Final punctuation
614 Pi Initial punctuation
615 Po Other punctuation
616 Ps Open punctuation
617
618 S Symbol
619 Sc Currency symbol
620 Sk Modifier symbol
621 Sm Mathematical symbol
622 So Other symbol
623
624 Z Separator
625 Zl Line separator
626 Zp Paragraph separator
627 Zs Space separator
628 </pre>
629 The special property L& is also supported: it matches a character that has
630 the Lu, Ll, or Lt property, in other words, a letter that is not classified as
631 a modifier or "other".
632 </P>
633 <P>
634 The Cs (Surrogate) property applies only to characters in the range U+D800 to
635 U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
636 cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
637 (see the discussion of PCRE_NO_UTF8_CHECK in the
638 <a href="pcreapi.html"><b>pcreapi</b></a>
639 page).
640 </P>
641 <P>
642 The long synonyms for these properties that Perl supports (such as \p{Letter})
643 are not supported by PCRE, nor is it permitted to prefix any of these
644 properties with "Is".
645 </P>
646 <P>
647 No character that is in the Unicode table has the Cn (unassigned) property.
648 Instead, this property is assumed for any code point that is not in the
649 Unicode table.
650 </P>
651 <P>
652 Specifying caseless matching does not affect these escape sequences. For
653 example, \p{Lu} always matches only upper case letters.
654 </P>
655 <P>
656 The \X escape matches any number of Unicode characters that form an extended
657 Unicode sequence. \X is equivalent to
658 <pre>
659 (?&#62;\PM\pM*)
660 </pre>
661 That is, it matches a character without the "mark" property, followed by zero
662 or more characters with the "mark" property, and treats the sequence as an
663 atomic group
664 <a href="#atomicgroup">(see below).</a>
665 Characters with the "mark" property are typically accents that affect the
666 preceding character. None of them have codepoints less than 256, so in
667 non-UTF-8 mode \X matches any one character.
668 </P>
669 <P>
670 Matching characters by Unicode property is not fast, because PCRE has to search
671 a structure that contains data for over fifteen thousand characters. That is
672 why the traditional escape sequences such as \d and \w do not use Unicode
673 properties in PCRE.
674 <a name="resetmatchstart"></a></P>
675 <br><b>
676 Resetting the match start
677 </b><br>
678 <P>
679 The escape sequence \K, which is a Perl 5.10 feature, causes any previously
680 matched characters not to be included in the final matched sequence. For
681 example, the pattern:
682 <pre>
683 foo\Kbar
684 </pre>
685 matches "foobar", but reports that it has matched "bar". This feature is
686 similar to a lookbehind assertion
687 <a href="#lookbehind">(described below).</a>
688 However, in this case, the part of the subject before the real match does not
689 have to be of fixed length, as lookbehind assertions do. The use of \K does
690 not interfere with the setting of
691 <a href="#subpattern">captured substrings.</a>
692 For example, when the pattern
693 <pre>
694 (foo)\Kbar
695 </pre>
696 matches "foobar", the first substring is still set to "foo".
697 <a name="smallassertions"></a></P>
698 <br><b>
699 Simple assertions
700 </b><br>
701 <P>
702 The final use of backslash is for certain simple assertions. An assertion
703 specifies a condition that has to be met at a particular point in a match,
704 without consuming any characters from the subject string. The use of
705 subpatterns for more complicated assertions is described
706 <a href="#bigassertions">below.</a>
707 The backslashed assertions are:
708 <pre>
709 \b matches at a word boundary
710 \B matches when not at a word boundary
711 \A matches at the start of the subject
712 \Z matches at the end of the subject
713 also matches before a newline at the end of the subject
714 \z matches only at the end of the subject
715 \G matches at the first matching position in the subject
716 </pre>
717 These assertions may not appear in character classes (but note that \b has a
718 different meaning, namely the backspace character, inside a character class).
719 </P>
720 <P>
721 A word boundary is a position in the subject string where the current character
722 and the previous character do not both match \w or \W (i.e. one matches
723 \w and the other matches \W), or the start or end of the string if the
724 first or last character matches \w, respectively.
725 </P>
726 <P>
727 The \A, \Z, and \z assertions differ from the traditional circumflex and
728 dollar (described in the next section) in that they only ever match at the very
729 start and end of the subject string, whatever options are set. Thus, they are
730 independent of multiline mode. These three assertions are not affected by the
731 PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
732 circumflex and dollar metacharacters. However, if the <i>startoffset</i>
733 argument of <b>pcre_exec()</b> is non-zero, indicating that matching is to start
734 at a point other than the beginning of the subject, \A can never match. The
735 difference between \Z and \z is that \Z matches before a newline at the end
736 of the string as well as at the very end, whereas \z matches only at the end.
737 </P>
738 <P>
739 The \G assertion is true only when the current matching position is at the
740 start point of the match, as specified by the <i>startoffset</i> argument of
741 <b>pcre_exec()</b>. It differs from \A when the value of <i>startoffset</i> is
742 non-zero. By calling <b>pcre_exec()</b> multiple times with appropriate
743 arguments, you can mimic Perl's /g option, and it is in this kind of
744 implementation where \G can be useful.
745 </P>
746 <P>
747 Note, however, that PCRE's interpretation of \G, as the start of the current
748 match, is subtly different from Perl's, which defines it as the end of the
749 previous match. In Perl, these can be different when the previously matched
750 string was empty. Because PCRE does just one match at a time, it cannot
751 reproduce this behaviour.
752 </P>
753 <P>
754 If all the alternatives of a pattern begin with \G, the expression is anchored
755 to the starting match position, and the "anchored" flag is set in the compiled
756 regular expression.
757 </P>
758 <br><a name="SEC5" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>
759 <P>
760 Outside a character class, in the default matching mode, the circumflex
761 character is an assertion that is true only if the current matching point is
762 at the start of the subject string. If the <i>startoffset</i> argument of
763 <b>pcre_exec()</b> is non-zero, circumflex can never match if the PCRE_MULTILINE
764 option is unset. Inside a character class, circumflex has an entirely different
765 meaning
766 <a href="#characterclass">(see below).</a>
767 </P>
768 <P>
769 Circumflex need not be the first character of the pattern if a number of
770 alternatives are involved, but it should be the first thing in each alternative
771 in which it appears if the pattern is ever to match that branch. If all
772 possible alternatives start with a circumflex, that is, if the pattern is
773 constrained to match only at the start of the subject, it is said to be an
774 "anchored" pattern. (There are also other constructs that can cause a pattern
775 to be anchored.)
776 </P>
777 <P>
778 A dollar character is an assertion that is true only if the current matching
779 point is at the end of the subject string, or immediately before a newline
780 at the end of the string (by default). Dollar need not be the last character of
781 the pattern if a number of alternatives are involved, but it should be the last
782 item in any branch in which it appears. Dollar has no special meaning in a
783 character class.
784 </P>
785 <P>
786 The meaning of dollar can be changed so that it matches only at the very end of
787 the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
788 does not affect the \Z assertion.
789 </P>
790 <P>
791 The meanings of the circumflex and dollar characters are changed if the
792 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
793 immediately after internal newlines as well as at the start of the subject
794 string. It does not match after a newline that ends the string. A dollar
795 matches before any newlines in the string, as well as at the very end, when
796 PCRE_MULTILINE is set. When newline is specified as the two-character
797 sequence CRLF, isolated CR and LF characters do not indicate newlines.
798 </P>
799 <P>
800 For example, the pattern /^abc$/ matches the subject string "def\nabc" (where
801 \n represents a newline) in multiline mode, but not otherwise. Consequently,
802 patterns that are anchored in single line mode because all branches start with
803 ^ are not anchored in multiline mode, and a match for circumflex is possible
804 when the <i>startoffset</i> argument of <b>pcre_exec()</b> is non-zero. The
805 PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
806 </P>
807 <P>
808 Note that the sequences \A, \Z, and \z can be used to match the start and
809 end of the subject in both modes, and if all branches of a pattern start with
810 \A it is always anchored, whether or not PCRE_MULTILINE is set.
811 </P>
812 <br><a name="SEC6" href="#TOC1">FULL STOP (PERIOD, DOT)</a><br>
813 <P>
814 Outside a character class, a dot in the pattern matches any one character in
815 the subject string except (by default) a character that signifies the end of a
816 line. In UTF-8 mode, the matched character may be more than one byte long.
817 </P>
818 <P>
819 When a line ending is defined as a single character, dot never matches that
820 character; when the two-character sequence CRLF is used, dot does not match CR
821 if it is immediately followed by LF, but otherwise it matches all characters
822 (including isolated CRs and LFs). When any Unicode line endings are being
823 recognized, dot does not match CR or LF or any of the other line ending
824 characters.
825 </P>
826 <P>
827 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
828 option is set, a dot matches any one character, without exception. If the
829 two-character sequence CRLF is present in the subject string, it takes two dots
830 to match it.
831 </P>
832 <P>
833 The handling of dot is entirely independent of the handling of circumflex and
834 dollar, the only relationship being that they both involve newlines. Dot has no
835 special meaning in a character class.
836 </P>
837 <br><a name="SEC7" href="#TOC1">MATCHING A SINGLE BYTE</a><br>
838 <P>
839 Outside a character class, the escape sequence \C matches any one byte, both
840 in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
841 characters. The feature is provided in Perl in order to match individual bytes
842 in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
843 what remains in the string may be a malformed UTF-8 string. For this reason,
844 the \C escape sequence is best avoided.
845 </P>
846 <P>
847 PCRE does not allow \C to appear in lookbehind assertions
848 <a href="#lookbehind">(described below),</a>
849 because in UTF-8 mode this would make it impossible to calculate the length of
850 the lookbehind.
851 <a name="characterclass"></a></P>
852 <br><a name="SEC8" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
853 <P>
854 An opening square bracket introduces a character class, terminated by a closing
855 square bracket. A closing square bracket on its own is not special. If a
856 closing square bracket is required as a member of the class, it should be the
857 first data character in the class (after an initial circumflex, if present) or
858 escaped with a backslash.
859 </P>
860 <P>
861 A character class matches a single character in the subject. In UTF-8 mode, the
862 character may occupy more than one byte. A matched character must be in the set
863 of characters defined by the class, unless the first character in the class
864 definition is a circumflex, in which case the subject character must not be in
865 the set defined by the class. If a circumflex is actually required as a member
866 of the class, ensure it is not the first character, or escape it with a
867 backslash.
868 </P>
869 <P>
870 For example, the character class [aeiou] matches any lower case vowel, while
871 [^aeiou] matches any character that is not a lower case vowel. Note that a
872 circumflex is just a convenient notation for specifying the characters that
873 are in the class by enumerating those that are not. A class that starts with a
874 circumflex is not an assertion: it still consumes a character from the subject
875 string, and therefore it fails if the current pointer is at the end of the
876 string.
877 </P>
878 <P>
879 In UTF-8 mode, characters with values greater than 255 can be included in a
880 class as a literal string of bytes, or by using the \x{ escaping mechanism.
881 </P>
882 <P>
883 When caseless matching is set, any letters in a class represent both their
884 upper case and lower case versions, so for example, a caseless [aeiou] matches
885 "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
886 caseful version would. In UTF-8 mode, PCRE always understands the concept of
887 case for characters whose values are less than 128, so caseless matching is
888 always possible. For characters with higher values, the concept of case is
889 supported if PCRE is compiled with Unicode property support, but not otherwise.
890 If you want to use caseless matching for characters 128 and above, you must
891 ensure that PCRE is compiled with Unicode property support as well as with
892 UTF-8 support.
893 </P>
894 <P>
895 Characters that might indicate line breaks are never treated in any special way
896 when matching character classes, whatever line-ending sequence is in use, and
897 whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
898 such as [^a] always matches one of these characters.
899 </P>
900 <P>
901 The minus (hyphen) character can be used to specify a range of characters in a
902 character class. For example, [d-m] matches any letter between d and m,
903 inclusive. If a minus character is required in a class, it must be escaped with
904 a backslash or appear in a position where it cannot be interpreted as
905 indicating a range, typically as the first or last character in the class.
906 </P>
907 <P>
908 It is not possible to have the literal character "]" as the end character of a
909 range. A pattern such as [W-]46] is interpreted as a class of two characters
910 ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
911 "-46]". However, if the "]" is escaped with a backslash it is interpreted as
912 the end of range, so [W-\]46] is interpreted as a class containing a range
913 followed by two other characters. The octal or hexadecimal representation of
914 "]" can also be used to end a range.
915 </P>
916 <P>
917 Ranges operate in the collating sequence of character values. They can also be
918 used for characters specified numerically, for example [\000-\037]. In UTF-8
919 mode, ranges can include characters whose values are greater than 255, for
920 example [\x{100}-\x{2ff}].
921 </P>
922 <P>
923 If a range that includes letters is used when caseless matching is set, it
924 matches the letters in either case. For example, [W-c] is equivalent to
925 [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
926 tables for a French locale are in use, [\xc8-\xcb] matches accented E
927 characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
928 characters with values greater than 128 only when it is compiled with Unicode
929 property support.
930 </P>
931 <P>
932 The character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear
933 in a character class, and add the characters that they match to the class. For
934 example, [\dABCDEF] matches any hexadecimal digit. A circumflex can
935 conveniently be used with the upper case character types to specify a more
936 restricted set of characters than the matching lower case type. For example,
937 the class [^\W_] matches any letter or digit, but not underscore.
938 </P>
939 <P>
940 The only metacharacters that are recognized in character classes are backslash,
941 hyphen (only where it can be interpreted as specifying a range), circumflex
942 (only at the start), opening square bracket (only when it can be interpreted as
943 introducing a POSIX class name - see the next section), and the terminating
944 closing square bracket. However, escaping other non-alphanumeric characters
945 does no harm.
946 </P>
947 <br><a name="SEC9" href="#TOC1">POSIX CHARACTER CLASSES</a><br>
948 <P>
949 Perl supports the POSIX notation for character classes. This uses names
950 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
951 this notation. For example,
952 <pre>
953 [01[:alpha:]%]
954 </pre>
955 matches "0", "1", any alphabetic character, or "%". The supported class names
956 are
957 <pre>
958 alnum letters and digits
959 alpha letters
960 ascii character codes 0 - 127
961 blank space or tab only
962 cntrl control characters
963 digit decimal digits (same as \d)
964 graph printing characters, excluding space
965 lower lower case letters
966 print printing characters, including space
967 punct printing characters, excluding letters and digits
968 space white space (not quite the same as \s)
969 upper upper case letters
970 word "word" characters (same as \w)
971 xdigit hexadecimal digits
972 </pre>
973 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
974 space (32). Notice that this list includes the VT character (code 11). This
975 makes "space" different to \s, which does not include VT (for Perl
976 compatibility).
977 </P>
978 <P>
979 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
980 5.8. Another Perl extension is negation, which is indicated by a ^ character
981 after the colon. For example,
982 <pre>
983 [12[:^digit:]]
984 </pre>
985 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
986 syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
987 supported, and an error is given if they are encountered.
988 </P>
989 <P>
990 In UTF-8 mode, characters with values greater than 128 do not match any of
991 the POSIX character classes.
992 </P>
993 <br><a name="SEC10" href="#TOC1">VERTICAL BAR</a><br>
994 <P>
995 Vertical bar characters are used to separate alternative patterns. For example,
996 the pattern
997 <pre>
998 gilbert|sullivan
999 </pre>
1000 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1001 and an empty alternative is permitted (matching the empty string). The matching
1002 process tries each alternative in turn, from left to right, and the first one
1003 that succeeds is used. If the alternatives are within a subpattern
1004 <a href="#subpattern">(defined below),</a>
1005 "succeeds" means matching the rest of the main pattern as well as the
1006 alternative in the subpattern.
1007 </P>
1008 <br><a name="SEC11" href="#TOC1">INTERNAL OPTION SETTING</a><br>
1009 <P>
1010 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1011 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1012 the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1013 The option letters are
1014 <pre>
1015 i for PCRE_CASELESS
1016 m for PCRE_MULTILINE
1017 s for PCRE_DOTALL
1018 x for PCRE_EXTENDED
1019 </pre>
1020 For example, (?im) sets caseless, multiline matching. It is also possible to
1021 unset these options by preceding the letter with a hyphen, and a combined
1022 setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1023 PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1024 permitted. If a letter appears both before and after the hyphen, the option is
1025 unset.
1026 </P>
1027 <P>
1028 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1029 changed in the same way as the Perl-compatible options by using the characters
1030 J, U and X respectively.
1031 </P>
1032 <P>
1033 When an option change occurs at top level (that is, not inside subpattern
1034 parentheses), the change applies to the remainder of the pattern that follows.
1035 If the change is placed right at the start of a pattern, PCRE extracts it into
1036 the global options (and it will therefore show up in data extracted by the
1037 <b>pcre_fullinfo()</b> function).
1038 </P>
1039 <P>
1040 An option change within a subpattern (see below for a description of
1041 subpatterns) affects only that part of the current pattern that follows it, so
1042 <pre>
1043 (a(?i)b)c
1044 </pre>
1045 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1046 By this means, options can be made to have different settings in different
1047 parts of the pattern. Any changes made in one alternative do carry on
1048 into subsequent branches within the same subpattern. For example,
1049 <pre>
1050 (a(?i)b|c)
1051 </pre>
1052 matches "ab", "aB", "c", and "C", even though when matching "C" the first
1053 branch is abandoned before the option setting. This is because the effects of
1054 option settings happen at compile time. There would be some very weird
1055 behaviour otherwise.
1056 </P>
1057 <P>
1058 <b>Note:</b> There are other PCRE-specific options that can be set by the
1059 application when the compile or match functions are called. In some cases the
1060 pattern can contain special leading sequences to override what the application
1061 has set or what has been defaulted. Details are given in the section entitled
1062 <a href="#newlineseq">"Newline sequences"</a>
1063 above.
1064 <a name="subpattern"></a></P>
1065 <br><a name="SEC12" href="#TOC1">SUBPATTERNS</a><br>
1066 <P>
1067 Subpatterns are delimited by parentheses (round brackets), which can be nested.
1068 Turning part of a pattern into a subpattern does two things:
1069 <br>
1070 <br>
1071 1. It localizes a set of alternatives. For example, the pattern
1072 <pre>
1073 cat(aract|erpillar|)
1074 </pre>
1075 matches one of the words "cat", "cataract", or "caterpillar". Without the
1076 parentheses, it would match "cataract", "erpillar" or an empty string.
1077 <br>
1078 <br>
1079 2. It sets up the subpattern as a capturing subpattern. This means that, when
1080 the whole pattern matches, that portion of the subject string that matched the
1081 subpattern is passed back to the caller via the <i>ovector</i> argument of
1082 <b>pcre_exec()</b>. Opening parentheses are counted from left to right (starting
1083 from 1) to obtain numbers for the capturing subpatterns.
1084 </P>
1085 <P>
1086 For example, if the string "the red king" is matched against the pattern
1087 <pre>
1088 the ((red|white) (king|queen))
1089 </pre>
1090 the captured substrings are "red king", "red", and "king", and are numbered 1,
1091 2, and 3, respectively.
1092 </P>
1093 <P>
1094 The fact that plain parentheses fulfil two functions is not always helpful.
1095 There are often times when a grouping subpattern is required without a
1096 capturing requirement. If an opening parenthesis is followed by a question mark
1097 and a colon, the subpattern does not do any capturing, and is not counted when
1098 computing the number of any subsequent capturing subpatterns. For example, if
1099 the string "the white queen" is matched against the pattern
1100 <pre>
1101 the ((?:red|white) (king|queen))
1102 </pre>
1103 the captured substrings are "white queen" and "queen", and are numbered 1 and
1104 2. The maximum number of capturing subpatterns is 65535.
1105 </P>
1106 <P>
1107 As a convenient shorthand, if any option settings are required at the start of
1108 a non-capturing subpattern, the option letters may appear between the "?" and
1109 the ":". Thus the two patterns
1110 <pre>
1111 (?i:saturday|sunday)
1112 (?:(?i)saturday|sunday)
1113 </pre>
1114 match exactly the same set of strings. Because alternative branches are tried
1115 from left to right, and options are not reset until the end of the subpattern
1116 is reached, an option setting in one branch does affect subsequent branches, so
1117 the above patterns match "SUNDAY" as well as "Saturday".
1118 </P>
1119 <br><a name="SEC13" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br>
1120 <P>
1121 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1122 the same numbers for its capturing parentheses. Such a subpattern starts with
1123 (?| and is itself a non-capturing subpattern. For example, consider this
1124 pattern:
1125 <pre>
1126 (?|(Sat)ur|(Sun))day
1127 </pre>
1128 Because the two alternatives are inside a (?| group, both sets of capturing
1129 parentheses are numbered one. Thus, when the pattern matches, you can look
1130 at captured substring number one, whichever alternative matched. This construct
1131 is useful when you want to capture part, but not all, of one of a number of
1132 alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1133 number is reset at the start of each branch. The numbers of any capturing
1134 buffers that follow the subpattern start after the highest number used in any
1135 branch. The following example is taken from the Perl documentation.
1136 The numbers underneath show in which buffer the captured content will be
1137 stored.
1138 <pre>
1139 # before ---------------branch-reset----------- after
1140 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1141 # 1 2 2 3 2 3 4
1142 </pre>
1143 A backreference or a recursive call to a numbered subpattern always refers to
1144 the first one in the pattern with the given number.
1145 </P>
1146 <P>
1147 An alternative approach to using this "branch reset" feature is to use
1148 duplicate named subpatterns, as described in the next section.
1149 </P>
1150 <br><a name="SEC14" href="#TOC1">NAMED SUBPATTERNS</a><br>
1151 <P>
1152 Identifying capturing parentheses by number is simple, but it can be very hard
1153 to keep track of the numbers in complicated regular expressions. Furthermore,
1154 if an expression is modified, the numbers may change. To help with this
1155 difficulty, PCRE supports the naming of subpatterns. This feature was not
1156 added to Perl until release 5.10. Python had the feature earlier, and PCRE
1157 introduced it at release 4.0, using the Python syntax. PCRE now supports both
1158 the Perl and the Python syntax.
1159 </P>
1160 <P>
1161 In PCRE, a subpattern can be named in one of three ways: (?&#60;name&#62;...) or
1162 (?'name'...) as in Perl, or (?P&#60;name&#62;...) as in Python. References to capturing
1163 parentheses from other parts of the pattern, such as
1164 <a href="#backreferences">backreferences,</a>
1165 <a href="#recursion">recursion,</a>
1166 and
1167 <a href="#conditions">conditions,</a>
1168 can be made by name as well as by number.
1169 </P>
1170 <P>
1171 Names consist of up to 32 alphanumeric characters and underscores. Named
1172 capturing parentheses are still allocated numbers as well as names, exactly as
1173 if the names were not present. The PCRE API provides function calls for
1174 extracting the name-to-number translation table from a compiled pattern. There
1175 is also a convenience function for extracting a captured substring by name.
1176 </P>
1177 <P>
1178 By default, a name must be unique within a pattern, but it is possible to relax
1179 this constraint by setting the PCRE_DUPNAMES option at compile time. This can
1180 be useful for patterns where only one instance of the named parentheses can
1181 match. Suppose you want to match the name of a weekday, either as a 3-letter
1182 abbreviation or as the full name, and in both cases you want to extract the
1183 abbreviation. This pattern (ignoring the line breaks) does the job:
1184 <pre>
1185 (?&#60;DN&#62;Mon|Fri|Sun)(?:day)?|
1186 (?&#60;DN&#62;Tue)(?:sday)?|
1187 (?&#60;DN&#62;Wed)(?:nesday)?|
1188 (?&#60;DN&#62;Thu)(?:rsday)?|
1189 (?&#60;DN&#62;Sat)(?:urday)?
1190 </pre>
1191 There are five capturing substrings, but only one is ever set after a match.
1192 (An alternative way of solving this problem is to use a "branch reset"
1193 subpattern, as described in the previous section.)
1194 </P>
1195 <P>
1196 The convenience function for extracting the data by name returns the substring
1197 for the first (and in this example, the only) subpattern of that name that
1198 matched. This saves searching to find which numbered subpattern it was. If you
1199 make a reference to a non-unique named subpattern from elsewhere in the
1200 pattern, the one that corresponds to the lowest number is used. For further
1201 details of the interfaces for handling named subpatterns, see the
1202 <a href="pcreapi.html"><b>pcreapi</b></a>
1203 documentation.
1204 </P>
1205 <P>
1206 <b>Warning:</b> You cannot use different names to distinguish between two
1207 subpatterns with the same number (see the previous section) because PCRE uses
1208 only the numbers when matching.
1209 </P>
1210 <br><a name="SEC15" href="#TOC1">REPETITION</a><br>
1211 <P>
1212 Repetition is specified by quantifiers, which can follow any of the following
1213 items:
1214 <pre>
1215 a literal data character
1216 the dot metacharacter
1217 the \C escape sequence
1218 the \X escape sequence (in UTF-8 mode with Unicode properties)
1219 the \R escape sequence
1220 an escape such as \d that matches a single character
1221 a character class
1222 a back reference (see next section)
1223 a parenthesized subpattern (unless it is an assertion)
1224 </pre>
1225 The general repetition quantifier specifies a minimum and maximum number of
1226 permitted matches, by giving the two numbers in curly brackets (braces),
1227 separated by a comma. The numbers must be less than 65536, and the first must
1228 be less than or equal to the second. For example:
1229 <pre>
1230 z{2,4}
1231 </pre>
1232 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1233 character. If the second number is omitted, but the comma is present, there is
1234 no upper limit; if the second number and the comma are both omitted, the
1235 quantifier specifies an exact number of required matches. Thus
1236 <pre>
1237 [aeiou]{3,}
1238 </pre>
1239 matches at least 3 successive vowels, but may match many more, while
1240 <pre>
1241 \d{8}
1242 </pre>
1243 matches exactly 8 digits. An opening curly bracket that appears in a position
1244 where a quantifier is not allowed, or one that does not match the syntax of a
1245 quantifier, is taken as a literal character. For example, {,6} is not a
1246 quantifier, but a literal string of four characters.
1247 </P>
1248 <P>
1249 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
1250 bytes. Thus, for example, \x{100}{2} matches two UTF-8 characters, each of
1251 which is represented by a two-byte sequence. Similarly, when Unicode property
1252 support is available, \X{3} matches three Unicode extended sequences, each of
1253 which may be several bytes long (and they may be of different lengths).
1254 </P>
1255 <P>
1256 The quantifier {0} is permitted, causing the expression to behave as if the
1257 previous item and the quantifier were not present. This may be useful for
1258 subpatterns that are referenced as
1259 <a href="#subpatternsassubroutines">subroutines</a>
1260 from elsewhere in the pattern. Items other than subpatterns that have a {0}
1261 quantifier are omitted from the compiled pattern.
1262 </P>
1263 <P>
1264 For convenience, the three most common quantifiers have single-character
1265 abbreviations:
1266 <pre>
1267 * is equivalent to {0,}
1268 + is equivalent to {1,}
1269 ? is equivalent to {0,1}
1270 </pre>
1271 It is possible to construct infinite loops by following a subpattern that can
1272 match no characters with a quantifier that has no upper limit, for example:
1273 <pre>
1274 (a?)*
1275 </pre>
1276 Earlier versions of Perl and PCRE used to give an error at compile time for
1277 such patterns. However, because there are cases where this can be useful, such
1278 patterns are now accepted, but if any repetition of the subpattern does in fact
1279 match no characters, the loop is forcibly broken.
1280 </P>
1281 <P>
1282 By default, the quantifiers are "greedy", that is, they match as much as
1283 possible (up to the maximum number of permitted times), without causing the
1284 rest of the pattern to fail. The classic example of where this gives problems
1285 is in trying to match comments in C programs. These appear between /* and */
1286 and within the comment, individual * and / characters may appear. An attempt to
1287 match C comments by applying the pattern
1288 <pre>
1289 /\*.*\*/
1290 </pre>
1291 to the string
1292 <pre>
1293 /* first comment */ not comment /* second comment */
1294 </pre>
1295 fails, because it matches the entire string owing to the greediness of the .*
1296 item.
1297 </P>
1298 <P>
1299 However, if a quantifier is followed by a question mark, it ceases to be
1300 greedy, and instead matches the minimum number of times possible, so the
1301 pattern
1302 <pre>
1303 /\*.*?\*/
1304 </pre>
1305 does the right thing with the C comments. The meaning of the various
1306 quantifiers is not otherwise changed, just the preferred number of matches.
1307 Do not confuse this use of question mark with its use as a quantifier in its
1308 own right. Because it has two uses, it can sometimes appear doubled, as in
1309 <pre>
1310 \d??\d
1311 </pre>
1312 which matches one digit by preference, but can match two if that is the only
1313 way the rest of the pattern matches.
1314 </P>
1315 <P>
1316 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1317 the quantifiers are not greedy by default, but individual ones can be made
1318 greedy by following them with a question mark. In other words, it inverts the
1319 default behaviour.
1320 </P>
1321 <P>
1322 When a parenthesized subpattern is quantified with a minimum repeat count that
1323 is greater than 1 or with a limited maximum, more memory is required for the
1324 compiled pattern, in proportion to the size of the minimum or maximum.
1325 </P>
1326 <P>
1327 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1328 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1329 implicitly anchored, because whatever follows will be tried against every
1330 character position in the subject string, so there is no point in retrying the
1331 overall match at any position after the first. PCRE normally treats such a
1332 pattern as though it were preceded by \A.
1333 </P>
1334 <P>
1335 In cases where it is known that the subject string contains no newlines, it is
1336 worth setting PCRE_DOTALL in order to obtain this optimization, or
1337 alternatively using ^ to indicate anchoring explicitly.
1338 </P>
1339 <P>
1340 However, there is one situation where the optimization cannot be used. When .*
1341 is inside capturing parentheses that are the subject of a backreference
1342 elsewhere in the pattern, a match at the start may fail where a later one
1343 succeeds. Consider, for example:
1344 <pre>
1345 (.*)abc\1
1346 </pre>
1347 If the subject is "xyz123abc123" the match point is the fourth character. For
1348 this reason, such a pattern is not implicitly anchored.
1349 </P>
1350 <P>
1351 When a capturing subpattern is repeated, the value captured is the substring
1352 that matched the final iteration. For example, after
1353 <pre>
1354 (tweedle[dume]{3}\s*)+
1355 </pre>
1356 has matched "tweedledum tweedledee" the value of the captured substring is
1357 "tweedledee". However, if there are nested capturing subpatterns, the
1358 corresponding captured values may have been set in previous iterations. For
1359 example, after
1360 <pre>
1361 /(a|(b))+/
1362 </pre>
1363 matches "aba" the value of the second captured substring is "b".
1364 <a name="atomicgroup"></a></P>
1365 <br><a name="SEC16" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>
1366 <P>
1367 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1368 repetition, failure of what follows normally causes the repeated item to be
1369 re-evaluated to see if a different number of repeats allows the rest of the
1370 pattern to match. Sometimes it is useful to prevent this, either to change the
1371 nature of the match, or to cause it fail earlier than it otherwise might, when
1372 the author of the pattern knows there is no point in carrying on.
1373 </P>
1374 <P>
1375 Consider, for example, the pattern \d+foo when applied to the subject line
1376 <pre>
1377 123456bar
1378 </pre>
1379 After matching all 6 digits and then failing to match "foo", the normal
1380 action of the matcher is to try again with only 5 digits matching the \d+
1381 item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1382 (a term taken from Jeffrey Friedl's book) provides the means for specifying
1383 that once a subpattern has matched, it is not to be re-evaluated in this way.
1384 </P>
1385 <P>
1386 If we use atomic grouping for the previous example, the matcher gives up
1387 immediately on failing to match "foo" the first time. The notation is a kind of
1388 special parenthesis, starting with (?&#62; as in this example:
1389 <pre>
1390 (?&#62;\d+)foo
1391 </pre>
1392 This kind of parenthesis "locks up" the part of the pattern it contains once
1393 it has matched, and a failure further into the pattern is prevented from
1394 backtracking into it. Backtracking past it to previous items, however, works as
1395 normal.
1396 </P>
1397 <P>
1398 An alternative description is that a subpattern of this type matches the string
1399 of characters that an identical standalone pattern would match, if anchored at
1400 the current point in the subject string.
1401 </P>
1402 <P>
1403 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1404 the above example can be thought of as a maximizing repeat that must swallow
1405 everything it can. So, while both \d+ and \d+? are prepared to adjust the
1406 number of digits they match in order to make the rest of the pattern match,
1407 (?&#62;\d+) can only match an entire sequence of digits.
1408 </P>
1409 <P>
1410 Atomic groups in general can of course contain arbitrarily complicated
1411 subpatterns, and can be nested. However, when the subpattern for an atomic
1412 group is just a single repeated item, as in the example above, a simpler
1413 notation, called a "possessive quantifier" can be used. This consists of an
1414 additional + character following a quantifier. Using this notation, the
1415 previous example can be rewritten as
1416 <pre>
1417 \d++foo
1418 </pre>
1419 Note that a possessive quantifier can be used with an entire group, for
1420 example:
1421 <pre>
1422 (abc|xyz){2,3}+
1423 </pre>
1424 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1425 option is ignored. They are a convenient notation for the simpler forms of
1426 atomic group. However, there is no difference in the meaning of a possessive
1427 quantifier and the equivalent atomic group, though there may be a performance
1428 difference; possessive quantifiers should be slightly faster.
1429 </P>
1430 <P>
1431 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1432 Jeffrey Friedl originated the idea (and the name) in the first edition of his
1433 book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1434 package, and PCRE copied it from there. It ultimately found its way into Perl
1435 at release 5.10.
1436 </P>
1437 <P>
1438 PCRE has an optimization that automatically "possessifies" certain simple
1439 pattern constructs. For example, the sequence A+B is treated as A++B because
1440 there is no point in backtracking into a sequence of A's when B must follow.
1441 </P>
1442 <P>
1443 When a pattern contains an unlimited repeat inside a subpattern that can itself
1444 be repeated an unlimited number of times, the use of an atomic group is the
1445 only way to avoid some failing matches taking a very long time indeed. The
1446 pattern
1447 <pre>
1448 (\D+|&#60;\d+&#62;)*[!?]
1449 </pre>
1450 matches an unlimited number of substrings that either consist of non-digits, or
1451 digits enclosed in &#60;&#62;, followed by either ! or ?. When it matches, it runs
1452 quickly. However, if it is applied to
1453 <pre>
1454 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1455 </pre>
1456 it takes a long time before reporting failure. This is because the string can
1457 be divided between the internal \D+ repeat and the external * repeat in a
1458 large number of ways, and all have to be tried. (The example uses [!?] rather
1459 than a single character at the end, because both PCRE and Perl have an
1460 optimization that allows for fast failure when a single character is used. They
1461 remember the last single character that is required for a match, and fail early
1462 if it is not present in the string.) If the pattern is changed so that it uses
1463 an atomic group, like this:
1464 <pre>
1465 ((?&#62;\D+)|&#60;\d+&#62;)*[!?]
1466 </pre>
1467 sequences of non-digits cannot be broken, and failure happens quickly.
1468 <a name="backreferences"></a></P>
1469 <br><a name="SEC17" href="#TOC1">BACK REFERENCES</a><br>
1470 <P>
1471 Outside a character class, a backslash followed by a digit greater than 0 (and
1472 possibly further digits) is a back reference to a capturing subpattern earlier
1473 (that is, to its left) in the pattern, provided there have been that many
1474 previous capturing left parentheses.
1475 </P>
1476 <P>
1477 However, if the decimal number following the backslash is less than 10, it is
1478 always taken as a back reference, and causes an error only if there are not
1479 that many capturing left parentheses in the entire pattern. In other words, the
1480 parentheses that are referenced need not be to the left of the reference for
1481 numbers less than 10. A "forward back reference" of this type can make sense
1482 when a repetition is involved and the subpattern to the right has participated
1483 in an earlier iteration.
1484 </P>
1485 <P>
1486 It is not possible to have a numerical "forward back reference" to a subpattern
1487 whose number is 10 or more using this syntax because a sequence such as \50 is
1488 interpreted as a character defined in octal. See the subsection entitled
1489 "Non-printing characters"
1490 <a href="#digitsafterbackslash">above</a>
1491 for further details of the handling of digits following a backslash. There is
1492 no such problem when named parentheses are used. A back reference to any
1493 subpattern is possible using named parentheses (see below).
1494 </P>
1495 <P>
1496 Another way of avoiding the ambiguity inherent in the use of digits following a
1497 backslash is to use the \g escape sequence, which is a feature introduced in
1498 Perl 5.10. This escape must be followed by an unsigned number or a negative
1499 number, optionally enclosed in braces. These examples are all identical:
1500 <pre>
1501 (ring), \1
1502 (ring), \g1
1503 (ring), \g{1}
1504 </pre>
1505 An unsigned number specifies an absolute reference without the ambiguity that
1506 is present in the older syntax. It is also useful when literal digits follow
1507 the reference. A negative number is a relative reference. Consider this
1508 example:
1509 <pre>
1510 (abc(def)ghi)\g{-1}
1511 </pre>
1512 The sequence \g{-1} is a reference to the most recently started capturing
1513 subpattern before \g, that is, is it equivalent to \2. Similarly, \g{-2}
1514 would be equivalent to \1. The use of relative references can be helpful in
1515 long patterns, and also in patterns that are created by joining together
1516 fragments that contain references within themselves.
1517 </P>
1518 <P>
1519 A back reference matches whatever actually matched the capturing subpattern in
1520 the current subject string, rather than anything matching the subpattern
1521 itself (see
1522 <a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a>
1523 below for a way of doing that). So the pattern
1524 <pre>
1525 (sens|respons)e and \1ibility
1526 </pre>
1527 matches "sense and sensibility" and "response and responsibility", but not
1528 "sense and responsibility". If caseful matching is in force at the time of the
1529 back reference, the case of letters is relevant. For example,
1530 <pre>
1531 ((?i)rah)\s+\1
1532 </pre>
1533 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1534 capturing subpattern is matched caselessly.
1535 </P>
1536 <P>
1537 There are several different ways of writing back references to named
1538 subpatterns. The .NET syntax \k{name} and the Perl syntax \k&#60;name&#62; or
1539 \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1540 back reference syntax, in which \g can be used for both numeric and named
1541 references, is also supported. We could rewrite the above example in any of
1542 the following ways:
1543 <pre>
1544 (?&#60;p1&#62;(?i)rah)\s+\k&#60;p1&#62;
1545 (?'p1'(?i)rah)\s+\k{p1}
1546 (?P&#60;p1&#62;(?i)rah)\s+(?P=p1)
1547 (?&#60;p1&#62;(?i)rah)\s+\g{p1}
1548 </pre>
1549 A subpattern that is referenced by name may appear in the pattern before or
1550 after the reference.
1551 </P>
1552 <P>
1553 There may be more than one back reference to the same subpattern. If a
1554 subpattern has not actually been used in a particular match, any back
1555 references to it always fail. For example, the pattern
1556 <pre>
1557 (a|(bc))\2
1558 </pre>
1559 always fails if it starts to match "a" rather than "bc". Because there may be
1560 many capturing parentheses in a pattern, all digits following the backslash are
1561 taken as part of a potential back reference number. If the pattern continues
1562 with a digit character, some delimiter must be used to terminate the back
1563 reference. If the PCRE_EXTENDED option is set, this can be whitespace.
1564 Otherwise an empty comment (see
1565 <a href="#comments">"Comments"</a>
1566 below) can be used.
1567 </P>
1568 <P>
1569 A back reference that occurs inside the parentheses to which it refers fails
1570 when the subpattern is first used, so, for example, (a\1) never matches.
1571 However, such references can be useful inside repeated subpatterns. For
1572 example, the pattern
1573 <pre>
1574 (a|b\1)+
1575 </pre>
1576 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1577 the subpattern, the back reference matches the character string corresponding
1578 to the previous iteration. In order for this to work, the pattern must be such
1579 that the first iteration does not need to match the back reference. This can be
1580 done using alternation, as in the example above, or by a quantifier with a
1581 minimum of zero.
1582 <a name="bigassertions"></a></P>
1583 <br><a name="SEC18" href="#TOC1">ASSERTIONS</a><br>
1584 <P>
1585 An assertion is a test on the characters following or preceding the current
1586 matching point that does not actually consume any characters. The simple
1587 assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described
1588 <a href="#smallassertions">above.</a>
1589 </P>
1590 <P>
1591 More complicated assertions are coded as subpatterns. There are two kinds:
1592 those that look ahead of the current position in the subject string, and those
1593 that look behind it. An assertion subpattern is matched in the normal way,
1594 except that it does not cause the current matching position to be changed.
1595 </P>
1596 <P>
1597 Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1598 because it makes no sense to assert the same thing several times. If any kind
1599 of assertion contains capturing subpatterns within it, these are counted for
1600 the purposes of numbering the capturing subpatterns in the whole pattern.
1601 However, substring capturing is carried out only for positive assertions,
1602 because it does not make sense for negative assertions.
1603 </P>
1604 <br><b>
1605 Lookahead assertions
1606 </b><br>
1607 <P>
1608 Lookahead assertions start with (?= for positive assertions and (?! for
1609 negative assertions. For example,
1610 <pre>
1611 \w+(?=;)
1612 </pre>
1613 matches a word followed by a semicolon, but does not include the semicolon in
1614 the match, and
1615 <pre>
1616 foo(?!bar)
1617 </pre>
1618 matches any occurrence of "foo" that is not followed by "bar". Note that the
1619 apparently similar pattern
1620 <pre>
1621 (?!foo)bar
1622 </pre>
1623 does not find an occurrence of "bar" that is preceded by something other than
1624 "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1625 (?!foo) is always true when the next three characters are "bar". A
1626 lookbehind assertion is needed to achieve the other effect.
1627 </P>
1628 <P>
1629 If you want to force a matching failure at some point in a pattern, the most
1630 convenient way to do it is with (?!) because an empty string always matches, so
1631 an assertion that requires there not to be an empty string must always fail.
1632 <a name="lookbehind"></a></P>
1633 <br><b>
1634 Lookbehind assertions
1635 </b><br>
1636 <P>
1637 Lookbehind assertions start with (?&#60;= for positive assertions and (?&#60;! for
1638 negative assertions. For example,
1639 <pre>
1640 (?&#60;!foo)bar
1641 </pre>
1642 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1643 a lookbehind assertion are restricted such that all the strings it matches must
1644 have a fixed length. However, if there are several top-level alternatives, they
1645 do not all have to have the same fixed length. Thus
1646 <pre>
1647 (?&#60;=bullock|donkey)
1648 </pre>
1649 is permitted, but
1650 <pre>
1651 (?&#60;!dogs?|cats?)
1652 </pre>
1653 causes an error at compile time. Branches that match different length strings
1654 are permitted only at the top level of a lookbehind assertion. This is an
1655 extension compared with Perl (at least for 5.8), which requires all branches to
1656 match the same length of string. An assertion such as
1657 <pre>
1658 (?&#60;=ab(c|de))
1659 </pre>
1660 is not permitted, because its single top-level branch can match two different
1661 lengths, but it is acceptable if rewritten to use two top-level branches:
1662 <pre>
1663 (?&#60;=abc|abde)
1664 </pre>
1665 In some cases, the Perl 5.10 escape sequence \K
1666 <a href="#resetmatchstart">(see above)</a>
1667 can be used instead of a lookbehind assertion; this is not restricted to a
1668 fixed-length.
1669 </P>
1670 <P>
1671 The implementation of lookbehind assertions is, for each alternative, to
1672 temporarily move the current position back by the fixed length and then try to
1673 match. If there are insufficient characters before the current position, the
1674 assertion fails.
1675 </P>
1676 <P>
1677 PCRE does not allow the \C escape (which matches a single byte in UTF-8 mode)
1678 to appear in lookbehind assertions, because it makes it impossible to calculate
1679 the length of the lookbehind. The \X and \R escapes, which can match
1680 different numbers of bytes, are also not permitted.
1681 </P>
1682 <P>
1683 Possessive quantifiers can be used in conjunction with lookbehind assertions to
1684 specify efficient matching at the end of the subject string. Consider a simple
1685 pattern such as
1686 <pre>
1687 abcd$
1688 </pre>
1689 when applied to a long string that does not match. Because matching proceeds
1690 from left to right, PCRE will look for each "a" in the subject and then see if
1691 what follows matches the rest of the pattern. If the pattern is specified as
1692 <pre>
1693 ^.*abcd$
1694 </pre>
1695 the initial .* matches the entire string at first, but when this fails (because
1696 there is no following "a"), it backtracks to match all but the last character,
1697 then all but the last two characters, and so on. Once again the search for "a"
1698 covers the entire string, from right to left, so we are no better off. However,
1699 if the pattern is written as
1700 <pre>
1701 ^.*+(?&#60;=abcd)
1702 </pre>
1703 there can be no backtracking for the .*+ item; it can match only the entire
1704 string. The subsequent lookbehind assertion does a single test on the last four
1705 characters. If it fails, the match fails immediately. For long strings, this
1706 approach makes a significant difference to the processing time.
1707 </P>
1708 <br><b>
1709 Using multiple assertions
1710 </b><br>
1711 <P>
1712 Several assertions (of any sort) may occur in succession. For example,
1713 <pre>
1714 (?&#60;=\d{3})(?&#60;!999)foo
1715 </pre>
1716 matches "foo" preceded by three digits that are not "999". Notice that each of
1717 the assertions is applied independently at the same point in the subject
1718 string. First there is a check that the previous three characters are all
1719 digits, and then there is a check that the same three characters are not "999".
1720 This pattern does <i>not</i> match "foo" preceded by six characters, the first
1721 of which are digits and the last three of which are not "999". For example, it
1722 doesn't match "123abcfoo". A pattern to do that is
1723 <pre>
1724 (?&#60;=\d{3}...)(?&#60;!999)foo
1725 </pre>
1726 This time the first assertion looks at the preceding six characters, checking
1727 that the first three are digits, and then the second assertion checks that the
1728 preceding three characters are not "999".
1729 </P>
1730 <P>
1731 Assertions can be nested in any combination. For example,
1732 <pre>
1733 (?&#60;=(?&#60;!foo)bar)baz
1734 </pre>
1735 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1736 preceded by "foo", while
1737 <pre>
1738 (?&#60;=\d{3}(?!999)...)foo
1739 </pre>
1740 is another pattern that matches "foo" preceded by three digits and any three
1741 characters that are not "999".
1742 <a name="conditions"></a></P>
1743 <br><a name="SEC19" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>
1744 <P>
1745 It is possible to cause the matching process to obey a subpattern
1746 conditionally or to choose between two alternative subpatterns, depending on
1747 the result of an assertion, or whether a previous capturing subpattern matched
1748 or not. The two possible forms of conditional subpattern are
1749 <pre>
1750 (?(condition)yes-pattern)
1751 (?(condition)yes-pattern|no-pattern)
1752 </pre>
1753 If the condition is satisfied, the yes-pattern is used; otherwise the
1754 no-pattern (if present) is used. If there are more than two alternatives in the
1755 subpattern, a compile-time error occurs.
1756 </P>
1757 <P>
1758 There are four kinds of condition: references to subpatterns, references to
1759 recursion, a pseudo-condition called DEFINE, and assertions.
1760 </P>
1761 <br><b>
1762 Checking for a used subpattern by number
1763 </b><br>
1764 <P>
1765 If the text between the parentheses consists of a sequence of digits, the
1766 condition is true if the capturing subpattern of that number has previously
1767 matched. An alternative notation is to precede the digits with a plus or minus
1768 sign. In this case, the subpattern number is relative rather than absolute.
1769 The most recently opened parentheses can be referenced by (?(-1), the next most
1770 recent by (?(-2), and so on. In looping constructs it can also make sense to
1771 refer to subsequent groups with constructs such as (?(+2).
1772 </P>
1773 <P>
1774 Consider the following pattern, which contains non-significant white space to
1775 make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1776 three parts for ease of discussion:
1777 <pre>
1778 ( \( )? [^()]+ (?(1) \) )
1779 </pre>
1780 The first part matches an optional opening parenthesis, and if that
1781 character is present, sets it as the first captured substring. The second part
1782 matches one or more characters that are not parentheses. The third part is a
1783 conditional subpattern that tests whether the first set of parentheses matched
1784 or not. If they did, that is, if subject started with an opening parenthesis,
1785 the condition is true, and so the yes-pattern is executed and a closing
1786 parenthesis is required. Otherwise, since no-pattern is not present, the
1787 subpattern matches nothing. In other words, this pattern matches a sequence of
1788 non-parentheses, optionally enclosed in parentheses.
1789 </P>
1790 <P>
1791 If you were embedding this pattern in a larger one, you could use a relative
1792 reference:
1793 <pre>
1794 ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
1795 </pre>
1796 This makes the fragment independent of the parentheses in the larger pattern.
1797 </P>
1798 <br><b>
1799 Checking for a used subpattern by name
1800 </b><br>
1801 <P>
1802 Perl uses the syntax (?(&#60;name&#62;)...) or (?('name')...) to test for a used
1803 subpattern by name. For compatibility with earlier versions of PCRE, which had
1804 this facility before Perl, the syntax (?(name)...) is also recognized. However,
1805 there is a possible ambiguity with this syntax, because subpattern names may
1806 consist entirely of digits. PCRE looks first for a named subpattern; if it
1807 cannot find one and the name consists entirely of digits, PCRE looks for a
1808 subpattern of that number, which must be greater than zero. Using subpattern
1809 names that consist entirely of digits is not recommended.
1810 </P>
1811 <P>
1812 Rewriting the above example to use a named subpattern gives this:
1813 <pre>
1814 (?&#60;OPEN&#62; \( )? [^()]+ (?(&#60;OPEN&#62;) \) )
1815
1816 </PRE>
1817 </P>
1818 <br><b>
1819 Checking for pattern recursion
1820 </b><br>
1821 <P>
1822 If the condition is the string (R), and there is no subpattern with the name R,
1823 the condition is true if a recursive call to the whole pattern or any
1824 subpattern has been made. If digits or a name preceded by ampersand follow the
1825 letter R, for example:
1826 <pre>
1827 (?(R3)...) or (?(R&name)...)
1828 </pre>
1829 the condition is true if the most recent recursion is into the subpattern whose
1830 number or name is given. This condition does not check the entire recursion
1831 stack.
1832 </P>
1833 <P>
1834 At "top level", all these recursion test conditions are false. Recursive
1835 patterns are described below.
1836 </P>
1837 <br><b>
1838 Defining subpatterns for use by reference only
1839 </b><br>
1840 <P>
1841 If the condition is the string (DEFINE), and there is no subpattern with the
1842 name DEFINE, the condition is always false. In this case, there may be only one
1843 alternative in the subpattern. It is always skipped if control reaches this
1844 point in the pattern; the idea of DEFINE is that it can be used to define
1845 "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
1846 is described below.) For example, a pattern to match an IPv4 address could be
1847 written like this (ignore whitespace and line breaks):
1848 <pre>
1849 (?(DEFINE) (?&#60;byte&#62; 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
1850 \b (?&byte) (\.(?&byte)){3} \b
1851 </pre>
1852 The first part of the pattern is a DEFINE group inside which a another group
1853 named "byte" is defined. This matches an individual component of an IPv4
1854 address (a number less than 256). When matching takes place, this part of the
1855 pattern is skipped because DEFINE acts like a false condition.
1856 </P>
1857 <P>
1858 The rest of the pattern uses references to the named group to match the four
1859 dot-separated components of an IPv4 address, insisting on a word boundary at
1860 each end.
1861 </P>
1862 <br><b>
1863 Assertion conditions
1864 </b><br>
1865 <P>
1866 If the condition is not in any of the above formats, it must be an assertion.
1867 This may be a positive or negative lookahead or lookbehind assertion. Consider
1868 this pattern, again containing non-significant white space, and with the two
1869 alternatives on the second line:
1870 <pre>
1871 (?(?=[^a-z]*[a-z])
1872 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
1873 </pre>
1874 The condition is a positive lookahead assertion that matches an optional
1875 sequence of non-letters followed by a letter. In other words, it tests for the
1876 presence of at least one letter in the subject. If a letter is found, the
1877 subject is matched against the first alternative; otherwise it is matched
1878 against the second. This pattern matches strings in one of the two forms
1879 dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
1880 <a name="comments"></a></P>
1881 <br><a name="SEC20" href="#TOC1">COMMENTS</a><br>
1882 <P>
1883 The sequence (?# marks the start of a comment that continues up to the next
1884 closing parenthesis. Nested parentheses are not permitted. The characters
1885 that make up a comment play no part in the pattern matching at all.
1886 </P>
1887 <P>
1888 If the PCRE_EXTENDED option is set, an unescaped # character outside a
1889 character class introduces a comment that continues to immediately after the
1890 next newline in the pattern.
1891 <a name="recursion"></a></P>
1892 <br><a name="SEC21" href="#TOC1">RECURSIVE PATTERNS</a><br>
1893 <P>
1894 Consider the problem of matching a string in parentheses, allowing for
1895 unlimited nested parentheses. Without the use of recursion, the best that can
1896 be done is to use a pattern that matches up to some fixed depth of nesting. It
1897 is not possible to handle an arbitrary nesting depth.
1898 </P>
1899 <P>
1900 For some time, Perl has provided a facility that allows regular expressions to
1901 recurse (amongst other things). It does this by interpolating Perl code in the
1902 expression at run time, and the code can refer to the expression itself. A Perl
1903 pattern using code interpolation to solve the parentheses problem can be
1904 created like this:
1905 <pre>
1906 $re = qr{\( (?: (?&#62;[^()]+) | (?p{$re}) )* \)}x;
1907 </pre>
1908 The (?p{...}) item interpolates Perl code at run time, and in this case refers
1909 recursively to the pattern in which it appears.
1910 </P>
1911 <P>
1912 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1913 supports special syntax for recursion of the entire pattern, and also for
1914 individual subpattern recursion. After its introduction in PCRE and Python,
1915 this kind of recursion was introduced into Perl at release 5.10.
1916 </P>
1917 <P>
1918 A special item that consists of (? followed by a number greater than zero and a
1919 closing parenthesis is a recursive call of the subpattern of the given number,
1920 provided that it occurs inside that subpattern. (If not, it is a "subroutine"
1921 call, which is described in the next section.) The special item (?R) or (?0) is
1922 a recursive call of the entire regular expression.
1923 </P>
1924 <P>
1925 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
1926 treated as an atomic group. That is, once it has matched some of the subject
1927 string, it is never re-entered, even if it contains untried alternatives and
1928 there is a subsequent matching failure.
1929 </P>
1930 <P>
1931 This PCRE pattern solves the nested parentheses problem (assume the
1932 PCRE_EXTENDED option is set so that white space is ignored):
1933 <pre>
1934 \( ( (?&#62;[^()]+) | (?R) )* \)
1935 </pre>
1936 First it matches an opening parenthesis. Then it matches any number of
1937 substrings which can either be a sequence of non-parentheses, or a recursive
1938 match of the pattern itself (that is, a correctly parenthesized substring).
1939 Finally there is a closing parenthesis.
1940 </P>
1941 <P>
1942 If this were part of a larger pattern, you would not want to recurse the entire
1943 pattern, so instead you could use this:
1944 <pre>
1945 ( \( ( (?&#62;[^()]+) | (?1) )* \) )
1946 </pre>
1947 We have put the pattern into parentheses, and caused the recursion to refer to
1948 them instead of the whole pattern.
1949 </P>
1950 <P>
1951 In a larger pattern, keeping track of parenthesis numbers can be tricky. This
1952 is made easier by the use of relative references. (A Perl 5.10 feature.)
1953 Instead of (?1) in the pattern above you can write (?-2) to refer to the second
1954 most recently opened parentheses preceding the recursion. In other words, a
1955 negative number counts capturing parentheses leftwards from the point at which
1956 it is encountered.
1957 </P>
1958 <P>
1959 It is also possible to refer to subsequently opened parentheses, by writing
1960 references such as (?+2). However, these cannot be recursive because the
1961 reference is not inside the parentheses that are referenced. They are always
1962 "subroutine" calls, as described in the next section.
1963 </P>
1964 <P>
1965 An alternative approach is to use named parentheses instead. The Perl syntax
1966 for this is (?&name); PCRE's earlier syntax (?P&#62;name) is also supported. We
1967 could rewrite the above example as follows:
1968 <pre>
1969 (?&#60;pn&#62; \( ( (?&#62;[^()]+) | (?&pn) )* \) )
1970 </pre>
1971 If there is more than one subpattern with the same name, the earliest one is
1972 used.
1973 </P>
1974 <P>
1975 This particular example pattern that we have been looking at contains nested
1976 unlimited repeats, and so the use of atomic grouping for matching strings of
1977 non-parentheses is important when applying the pattern to strings that do not
1978 match. For example, when this pattern is applied to
1979 <pre>
1980 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1981 </pre>
1982 it yields "no match" quickly. However, if atomic grouping is not used,
1983 the match runs for a very long time indeed because there are so many different
1984 ways the + and * repeats can carve up the subject, and all have to be tested
1985 before failure can be reported.
1986 </P>
1987 <P>
1988 At the end of a match, the values set for any capturing subpatterns are those
1989 from the outermost level of the recursion at which the subpattern value is set.
1990 If you want to obtain intermediate values, a callout function can be used (see
1991 below and the
1992 <a href="pcrecallout.html"><b>pcrecallout</b></a>
1993 documentation). If the pattern above is matched against
1994 <pre>
1995 (ab(cd)ef)
1996 </pre>
1997 the value for the capturing parentheses is "ef", which is the last value taken
1998 on at the top level. If additional parentheses are added, giving
1999 <pre>
2000 \( ( ( (?&#62;[^()]+) | (?R) )* ) \)
2001 ^ ^
2002 ^ ^
2003 </pre>
2004 the string they capture is "ab(cd)ef", the contents of the top level
2005 parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
2006 has to obtain extra memory to store data during a recursion, which it does by
2007 using <b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no
2008 memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2009 </P>
2010 <P>
2011 Do not confuse the (?R) item with the condition (R), which tests for recursion.
2012 Consider this pattern, which matches text in angle brackets, allowing for
2013 arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2014 recursing), whereas any characters are permitted at the outer level.
2015 <pre>
2016 &#60; (?: (?(R) \d++ | [^&#60;&#62;]*+) | (?R)) * &#62;
2017 </pre>
2018 In this pattern, (?(R) is the start of a conditional subpattern, with two
2019 different alternatives for the recursive and non-recursive cases. The (?R) item
2020 is the actual recursive call.
2021 <a name="subpatternsassubroutines"></a></P>
2022 <br><a name="SEC22" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>
2023 <P>
2024 If the syntax for a recursive subpattern reference (either by number or by
2025 name) is used outside the parentheses to which it refers, it operates like a
2026 subroutine in a programming language. The "called" subpattern may be defined
2027 before or after the reference. A numbered reference can be absolute or
2028 relative, as in these examples:
2029 <pre>
2030 (...(absolute)...)...(?2)...
2031 (...(relative)...)...(?-1)...
2032 (...(?+1)...(relative)...
2033 </pre>
2034 An earlier example pointed out that the pattern
2035 <pre>
2036 (sens|respons)e and \1ibility
2037 </pre>
2038 matches "sense and sensibility" and "response and responsibility", but not
2039 "sense and responsibility". If instead the pattern
2040 <pre>
2041 (sens|respons)e and (?1)ibility
2042 </pre>
2043 is used, it does match "sense and responsibility" as well as the other two
2044 strings. Another example is given in the discussion of DEFINE above.
2045 </P>
2046 <P>
2047 Like recursive subpatterns, a "subroutine" call is always treated as an atomic
2048 group. That is, once it has matched some of the subject string, it is never
2049 re-entered, even if it contains untried alternatives and there is a subsequent
2050 matching failure.
2051 </P>
2052 <P>
2053 When a subpattern is used as a subroutine, processing options such as
2054 case-independence are fixed when the subpattern is defined. They cannot be
2055 changed for different calls. For example, consider this pattern:
2056 <pre>
2057 (abc)(?i:(?-1))
2058 </pre>
2059 It matches "abcabc". It does not match "abcABC" because the change of
2060 processing option does not affect the called subpattern.
2061 <a name="onigurumasubroutines"></a></P>
2062 <br><a name="SEC23" href="#TOC1">ONIGURUMA SUBROUTINE SYNTAX</a><br>
2063 <P>
2064 For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
2065 a number enclosed either in angle brackets or single quotes, is an alternative
2066 syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2067 are two of the examples used above, rewritten using this syntax:
2068 <pre>
2069 (?&#60;pn&#62; \( ( (?&#62;[^()]+) | \g&#60;pn&#62; )* \) )
2070 (sens|respons)e and \g'1'ibility
2071 </pre>
2072 PCRE supports an extension to Oniguruma: if a number is preceded by a
2073 plus or a minus sign it is taken as a relative reference. For example:
2074 <pre>
2075 (abc)(?i:\g&#60;-1&#62;)
2076 </pre>
2077 Note that \g{...} (Perl syntax) and \g&#60;...&#62; (Oniguruma syntax) are <i>not</i>
2078 synonymous. The former is a back reference; the latter is a subroutine call.
2079 </P>
2080 <br><a name="SEC24" href="#TOC1">CALLOUTS</a><br>
2081 <P>
2082 Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2083 code to be obeyed in the middle of matching a regular expression. This makes it
2084 possible, amongst other things, to extract different substrings that match the
2085 same pair of parentheses when there is a repetition.
2086 </P>
2087 <P>
2088 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2089 code. The feature is called "callout". The caller of PCRE provides an external
2090 function by putting its entry point in the global variable <i>pcre_callout</i>.
2091 By default, this variable contains NULL, which disables all calling out.
2092 </P>
2093 <P>
2094 Within a regular expression, (?C) indicates the points at which the external
2095 function is to be called. If you want to identify different callout points, you
2096 can put a number less than 256 after the letter C. The default value is zero.
2097 For example, this pattern has two callout points:
2098 <pre>
2099 (?C1)abc(?C2)def
2100 </pre>
2101 If the PCRE_AUTO_CALLOUT flag is passed to <b>pcre_compile()</b>, callouts are
2102 automatically installed before each item in the pattern. They are all numbered
2103 255.
2104 </P>
2105 <P>
2106 During matching, when PCRE reaches a callout point (and <i>pcre_callout</i> is
2107 set), the external function is called. It is provided with the number of the
2108 callout, the position in the pattern, and, optionally, one item of data
2109 originally supplied by the caller of <b>pcre_exec()</b>. The callout function
2110 may cause matching to proceed, to backtrack, or to fail altogether. A complete
2111 description of the interface to the callout function is given in the
2112 <a href="pcrecallout.html"><b>pcrecallout</b></a>
2113 documentation.
2114 </P>
2115 <br><a name="SEC25" href="#TOC1">BACKTRACKING CONTROL</a><br>
2116 <P>
2117 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2118 are described in the Perl documentation as "experimental and subject to change
2119 or removal in a future version of Perl". It goes on to say: "Their usage in
2120 production code should be noted to avoid problems during upgrades." The same
2121 remarks apply to the PCRE features described in this section.
2122 </P>
2123 <P>
2124 Since these verbs are specifically related to backtracking, most of them can be
2125 used only when the pattern is to be matched using <b>pcre_exec()</b>, which uses
2126 a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2127 failing negative assertion, they cause an error if encountered by
2128 <b>pcre_dfa_exec()</b>.
2129 </P>
2130 <P>
2131 The new verbs make use of what was previously invalid syntax: an opening
2132 parenthesis followed by an asterisk. In Perl, they are generally of the form
2133 (*VERB:ARG) but PCRE does not support the use of arguments, so its general
2134 form is just (*VERB). Any number of these verbs may occur in a pattern. There
2135 are two kinds:
2136 </P>
2137 <br><b>
2138 Verbs that act immediately
2139 </b><br>
2140 <P>
2141 The following verbs act as soon as they are encountered:
2142 <pre>
2143 (*ACCEPT)
2144 </pre>
2145 This verb causes the match to end successfully, skipping the remainder of the
2146 pattern. When inside a recursion, only the innermost pattern is ended
2147 immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside
2148 capturing parentheses. In Perl, the data so far is captured: in PCRE no data is
2149 captured. For example:
2150 <pre>
2151 A(A|B(*ACCEPT)|C)D
2152 </pre>
2153 This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is
2154 captured.
2155 <pre>
2156 (*FAIL) or (*F)
2157 </pre>
2158 This verb causes the match to fail, forcing backtracking to occur. It is
2159 equivalent to (?!) but easier to read. The Perl documentation notes that it is
2160 probably useful only when combined with (?{}) or (??{}). Those are, of course,
2161 Perl features that are not present in PCRE. The nearest equivalent is the
2162 callout feature, as for example in this pattern:
2163 <pre>
2164 a+(?C)(*FAIL)
2165 </pre>
2166 A match with the string "aaaa" always fails, but the callout is taken before
2167 each backtrack happens (in this example, 10 times).
2168 </P>
2169 <br><b>
2170 Verbs that act after backtracking
2171 </b><br>
2172 <P>
2173 The following verbs do nothing when they are encountered. Matching continues
2174 with what follows, but if there is no subsequent match, a failure is forced.
2175 The verbs differ in exactly what kind of failure occurs.
2176 <pre>
2177 (*COMMIT)
2178 </pre>
2179 This verb causes the whole match to fail outright if the rest of the pattern
2180 does not match. Even if the pattern is unanchored, no further attempts to find
2181 a match by advancing the start point take place. Once (*COMMIT) has been
2182 passed, <b>pcre_exec()</b> is committed to finding a match at the current
2183 starting point, or not at all. For example:
2184 <pre>
2185 a+(*COMMIT)b
2186 </pre>
2187 This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2188 dynamic anchor, or "I've started, so I must finish."
2189 <pre>
2190 (*PRUNE)
2191 </pre>
2192 This verb causes the match to fail at the current position if the rest of the
2193 pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2194 advance to the next starting character then happens. Backtracking can occur as
2195 usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2196 if there is no match to the right, backtracking cannot cross (*PRUNE).
2197 In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2198 group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2199 be expressed in any other way.
2200 <pre>
2201 (*SKIP)
2202 </pre>
2203 This verb is like (*PRUNE), except that if the pattern is unanchored, the
2204 "bumpalong" advance is not to the next character, but to the position in the
2205 subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2206 was matched leading up to it cannot be part of a successful match. Consider:
2207 <pre>
2208 a+(*SKIP)b
2209 </pre>
2210 If the subject is "aaaac...", after the first match attempt fails (starting at
2211 the first character in the string), the starting point skips on to start the
2212 next attempt at "c". Note that a possessive quantifer does not have the same
2213 effect in this example; although it would suppress backtracking during the
2214 first match attempt, the second attempt would start at the second character
2215 instead of skipping on to "c".
2216 <pre>
2217 (*THEN)
2218 </pre>
2219 This verb causes a skip to the next alternation if the rest of the pattern does
2220 not match. That is, it cancels pending backtracking, but only within the
2221 current alternation. Its name comes from the observation that it can be used
2222 for a pattern-based if-then-else block:
2223 <pre>
2224 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2225 </pre>
2226 If the COND1 pattern matches, FOO is tried (and possibly further items after
2227 the end of the group if FOO succeeds); on failure the matcher skips to the
2228 second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2229 is used outside of any alternation, it acts exactly like (*PRUNE).
2230 </P>
2231 <br><a name="SEC26" href="#TOC1">SEE ALSO</a><br>
2232 <P>
2233 <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3), <b>pcre</b>(3).
2234 </P>
2235 <br><a name="SEC27" href="#TOC1">AUTHOR</a><br>
2236 <P>
2237 Philip Hazel
2238 <br>
2239 University Computing Service
2240 <br>
2241 Cambridge CB2 3QH, England.
2242 <br>
2243 </P>
2244 <br><a name="SEC28" href="#TOC1">REVISION</a><br>
2245 <P>
2246 Last updated: 08 March 2009
2247 <br>
2248 Copyright &copy; 1997-2009 University of Cambridge.
2249 <br>
2250 <p>
2251 Return to the <a href="index.html">PCRE index page</a>.
2252 </p>

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